ActiveRecord::RecordNotFoundのエラーメッセージを翻訳する ref: http://qiita.com/arakaji/items/7a7c262f35b17195d3d7

file0.rb

class UserController < ApplicationController

  def show
    begin
       @user = User.find(9999)
    rescue ActiveRecord::RecordNotFound => e
       # 翻訳処理を書く。   
    end
  end
end

file1.rb

def translate_record_not_found(error_msg)
        match = error_msg.match(/^Couldn't find ([\S].*) with '([\S]*)'=([\S].*)$/)

        # モデル名を翻訳する
        t_model = I18n.t('activerecord.models.' + match[1].underscore)
        # 属性名を翻訳する
        t_attr  = I18n.t('activerecord.attributes.' + match[1].underscore + '.' + match[2])

        I18n.t('errors.messages.not_found',
               { klass: t_model, attribute: t_attr, value: match[3] })

end

file2.txt

ja:
  activerecord:
    models:
      user: ユーザー
    attributes:
      user:
        id: "ID"

  errors:
    messages:
      not_found: "%{klass}の%{attribute}=`%{value}`は見つかりません"

file3.rb

class UserController < ApplicationController

  def show
    begin
       @user = User.find(9999)
    rescue ActiveRecord::RecordNotFound => e
       raise ActiveRecord::RecordNotFound, translate_record_not_found(error_msg)
    end
  end
end

ja.yml

ja:
  activerecord:
    models:
      user: ユーザー
    attributes:
      user:
        id: "ID"

  errors:
    messages:
      not_found: "%{klass}の%{attribute}=`%{value}`は見つかりません"